1. Field
Apparatuses and methods consistent with exemplary embodiments relate to magnetic resonance imaging (MRI), and, more particularly, to acquiring T2-weighted imaging data.
2. Description of the Related Art
When a substance such as human tissue is subjected to a uniform magnetic field, i.e., a static magnetic field B0, the individual magnetic moments of the excited nuclei in the tissue attempt to align with the static magnetic field B0, but precess about it in random order at their characteristic Larmor frequency. If the substance is subjected to a magnetic excitation field B1 that is in the x-y plane and that is near the Larmor frequency, the net magnetization aligned moment Mz may be rotated, i.e., tipped, into the x-y plane to generate a net transverse magnetic moment Mt. An MR signal is emitted by the excited nuclei, i.e., spins, after the excitation magnetic field B1 is terminated, and the MR signal may be received and processed to form an image.
In MRI systems, the excited spins induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is near the Larmor frequency, and its initial amplitude is determined by the magnitude of the transverse magnetic moment Mt. The amplitude of the emitted MR signal decays exponentially with time.
The amplitude of the MR signal is dependent on the spin-lattice relaxation process that is characterized by the time constant T1, i.e., a spin-lattice relaxation time. It describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization, i.e., z-magnetization. The difference in T1 values between tissues can be exploited to provide image contrast.
The T2 time constant is referred to as the spin-spin relaxation constant, or the transverse relaxation constant, and is characterized by a spin-spin relaxation time characterizing the signal decay. The T2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the excitation magnetic field B1 in a perfectly homogeneous magnetic field. The T1 time constant is much longer than T2 in most tissues of medical interest.
The biological tissues have different T2 values and this property may be exploited to enhance the contrast between the tissues. Accordingly, T2 serves as an informative MRI parameter, providing non-invasive measurements of tissue status and disease prognosis with respect to a wide range of applications and diseases, including discriminating between acute and chronic myocardial infarction. For example, quantitative T2 mapping may allow assessment of edema with less variability than T2-weighted imaging.
In order to quantify T2, multiple T2-weighted images may be acquired and fitted based on respective echo time (TE) lengths, assuming long repetition time (TR) for complete relaxation. In particular, related art T2 mapping methods acquire three images with different T2-weightings, for example, with T2 magnetization preparation time of 0 ms, 25 ms, and 55 ms. This data is then fit to a two-parameter model, to generate T2 maps. However, imperfection in RF pulses of a T2 magnetization preparation and application of additional RF pulses during imaging are not accounted for in the two-parameter model of the data fit process. Thus, the estimated T2 times may be inaccurate or not readily reproducible. That is, the two-parameter curve-fitting may mismatch the underlying image acquisition.
Accordingly, apparatuses and methods are needed to provide accurate T2 maps without extensive and/or impractical imaging sequences.